This invention relates to protective coatings. More specifically, it relates to coatings for preventing high temperature oxidation of the graphite electrodes of electric arc steelmaking furnaces.
Steelmaking by the electric arc furnace accounts for a considerable portion of the world' s steel production, especially of higher alloy and stainless steels. Because of its many advantages, such as its greater scheduling flexibility and its capability to produce practically all the known grades of steel including those with very low residual phosphorus and sulfur contents, the electric arc furnace process is receiving increased usage by steelmakers. The electrodes of electric arc steelmaking furnaces are made almost exclusively of graphite. Vast amounts of graphite are consumed during the steelmaking, the cost of electrode replacement being exceeded only by the cost of the steel scrap charge and the electrical power. A number of destructive mechanisms are responsible for the high electrode consumption and include sidewall oxidation, arc tip erosion, breakage, slag attack and hot metal attack. While little can be done to minimize the latter four mechanisms, development of an impermeable coating that will keep oxygen from reaching the graphite surface of the electrode would minimize and possibly eliminate the sidewall wear. It is therefore a primary object of this invention to provide such a protection in the form of an inexpensive and easily applied coating composition.
Many methods have been used to provide a suitable coating for the prevention of high-temperature oxidation of graphite electrodes and other graphite surfaces, these methods usually involving the formation of a silicon carbide layer. For example, in British Patent 866,818, silicon powder is applied and bonded to a graphite body such as by flame spraying; subsequent localized heating of the silicon is said to cause it to melt and simultaneously flow into and react with the graphite body to form a silicon carbide protective coating. In U.S. Pat. No. 3,275,471, a graphite component is dipped into a suspension of about equal amounts of finely divided silicon and silicon carbide, optionally with added carboxymethylcellulose, and subsequently fired at 1415.degree. to 1500.degree.C. for 5 to 25 seconds to produce a protective coating of silicon containing dispersed silicon carbide. And in British Patent 1,166,429, the graphite body is coated with a primary layer of silicon and a metallic surfacing layer of aluminum; preferably the layers are applied by flame spraying, and the body is heated to above 550.degree.C. to cause the components of the two layers to form a low-melting eutectic alloy and the silicon to react with the graphite body to form silicon carbide. An aqueous solution of alkali metal phosphate, borate or silicate may be applied to the surface layer to form a sealing layer. None of these methods, however, allows for the application of the protective coating to graphite electrodes in a simple and inexpensive manner, especially under the operating conditions of the electric arc steelmaking process.